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Does M5 rear sway bar help with turning the car

Hi All,

My 03 540i has the M-sport suspension but I don't think the sway bars are as thick as the ones on the M5 (15mm vs 16.5?)

I do not, or should I say have not yet, really driven the car hard and too fast but it is obvious to me the front wheels do not want to turn into turns as fast as I'd like (or in other words they seem slow to turn as compared to my other cars)

I sort of understand the basics of under/oversteer but do not really know how it specifically effects steering or how much a 1.5 - 2 mm effects it... so

Will a stiffer bar in the back help turn the car without making it too much to handle in regular & spirited back road driving? I do not want to push the balance so far to oversteer that the rear end wants to come around a lot.

I am more interested in steering effort then I am in stiffening up the body roll etc ...

If this rear bar upgrade is the hot ticket to fast steering, is the M5 16.5 bar the way to go, or, will the 17mm bar give that much more toward what I want??

Thanks again (and these questions have been put to you after I searched the archives ;-)

Yup (more)

I put an M5 rear bar on my 528 sport in 2004 and love it. It dials out a lot of the factory understeer and the improvement can be felt at all speeds.

You'll need new bushings to match the 16.5mm bar. Make sure you inspect all of your sway bar bushings and links at the same time. Weirdness or looseness in this fairly inexpensive system can make a huge difference in the stable, planted confidence of the car.

IMO, an M5 rear sway bar is a no-brainer. It's a piece of cake DIY too.

Re: Does M5 rear sway bar help with turning the car

My experience with moving from the "sport" bar to the M5 bar on my 540 was that it helped with the "lean" in the corners so that it "felt" more stable to me.

But I do not recall any change to the feeling of steering effort. The only time I have noticed that is when changing between various tires. A "summer performance" tire with an AA rating can require much more torque on the steering wheel than a standard AS touring tire.

But yes the heavier bar on the rear is worth it in my opinion (not a "huge" difference but noticeable to someone who is tuned to the handling issues.

When doing it I would also carefully inspect the 2 bushing clamps - they can tend to bend and it might be good to replace them - as well as the bushings.

M5 bar - why not from factory?

If the 16.5 mm bar helps, why didn't the factory install it when the car was shipped? Do some drivers complain or harshness or something? I always wonder about these simple upgrades any why they were not built like that.

thats exactly what it is. rought roads are felt >

more since a stiffer sway bar gives less independent motion between wheels on the same axle. The car will therefore rock side to side more over uneven surfaces. It slight when going from a 540i sport bar to an M5, but its noticeable.

Thats why they went with the dynamic antiroll bar option in the E60. A mechanism makes the connection stiffer as the car begins to pitch.

Must have a minimum amount of understeer

to satisfy the company's lawyers. While many of us prefer more neutral handling and can even deal with a little (or a lot of) oversteer, the average U.S. driver can be counted on to sue the company if the car they are driving goes off the road backwards. This is true even if the trunk is overloaded and the tires on the rear are bald and overinflated. So, the company engineers have to compensate for all of this by designing in plenty of understeer.
'03 525i Sapphire Blu/Grey, SP, Nav
'02 325it Orient Blu./Grey, SP, Nav
'98 323is Arctic Sil./Blk, SP
'95 M3 Cosmos Blk./Blk

+1, do it, you won't be disappointed, and if you (

really want to keep it flatter in the corners put the M5 front bar on...it's only a 6 hr job that requires disconnecting most of the front suspension, the steering box, and dropping the front subframe a few inches while you support the engine. It's a labor intensive job but I'd do it again. If you can find polyeurthane bushings for the rear bar that will help a bit too. I ended up machining some from a set for a Ford SUV for my car and K-Buds car about 5 or 6 years ago. They are still fine and don't squeek if you lube them with the synthetic grease that comes with them.

I doubt that anyone here drives the car hard (m)

enough on the street to envoke understeer. I really haven't noticed any difference under/over steering between OEM and the M5 bars. In theory there is some but tires, air pressure, springs, condition of struts all come into play. I like flat cornering, others may not.
I'd go with the fattest bar your wallet can afford.

If it does it's minimal. Your front tires and (m)

front suspension are responsible for steering and turn in response, the tires being the biggest contributor to quick response. Driving at the limit while cornering the bar may do something, but I doubt that most of us could tell the difference unless we measured the g force with both bars.
It may be slightly more noticable on an I6 that on our V8's because they have rack and pinion steering and a lighter engine.
This could turn into another "which oil is best" post..LOL

Re: +1, do it, you won't be disappointed, and if you (

The M5 bar is WAY more affordable than the Dinan bar and is only 0.5mm smaller. Like I said in my first post - no brainer.

You don't actually have to be sliding at a drift angle to feel the bar and it's reduction of the understeer. Even while still fully planted you can feel the tendancy of the car to turn or not. It's not necessarily "flatter", it just turns better.

I noticed a huge, immediate difference - very confidence inspiring and it felt like a mod giving me a real "advantage" on the road. Of course it's all blended in with the normal experience long ago but I can't imagine ever going back. The old bar is under my stairs just in case.

Since putting on the bar I've done a couple of emergency avoidance reactions in addition to sporty driving. One avoidance was really serious - stuff that has never come loose in the trunk and cabin ever before (no matter how sporty I was driving) flew all over. I basically had to cross my arms at 55mph on a three-lane width of asphalt and not go into the ditch on the other side. No problems with unanticipated oversteer. I have the incident burned into my memory in slow mo - the car, tires, everything behaved perfectly. The front and rear both developed slip angles with the slip helping the rear to come around a bit when I had to counter steer. If I had anything but my summer tires on (Toyo T1R at the time) things would have turned out different. The moral of this paragraph is that the M5 bar didn't go "too far" and is a great complement to the handling.

Maybe do not understand understeer vs. over

Possibly one of you can explain the issue of understeer versus neutral handling. I think I understand what understeer is. When I drove some of the most grim US cars in the 1980s and 1990s, like the Corsica, I would turn the steering wheel and the blasted body ploughed ahead straight. I kept wondering when the lousy car would turn. In contrast, my 1981 320i turns instantly and precisely. Turn the wheel and the body responds immediately. My 2002 530i is in-between (but much closer to the 320i model than the Corsica model). Two questions:

1. Wouldn't the safest car be the one where the driver's inputs to the steering wheel are immediate? Why do manufacturers want understeer? I just don't get it.

2. Should I install the 16.5 mm bar in my 530i and will that make more neutral handling? It has the regular suspension with 16 inch wheels, Michelin tires.

Re: Maybe do not understand understeer vs. over

Understeer is the tendency for the car to want to continue going straight. It doesn't steer enough, thus understeers.

Oversteer is the tendency for the car to want to spin. It steers too much, thus oversteers.

One catch is that you don't want to confuse over/understeer with steering response or turn in response. Your car can understeer but still respond immediately to steering input. If you haven't modified your BMW, it probably still understeers - badly - but the tires and steering system respond much more quickly than on your 80s domestic car. Shocking, I know :)

Further, there are still a lot of handling dynamics at play such as effective mass transfer depending on what else you're doing with the controls. If you do the right (wrong?) things, you can get a motorhome to oversteer and spin. Usually this involves anything which unsettles the chassis and makes the rear end light by either lifting from the throttle or stabbing at the brakes.

*THIS* is why engineers design so much understeer into a car. What do people do when a deer jumps out in front of them on the highway on a rainy night? Hit the brakes (lighten the rear) and yank at the wheel (unsettle the chassis) then yank it another direction (spin city).

These principles are another reason why people can spin if they go too fast on an onramp or similar. They start exceeding the traction limits (front or rear), they panic and lift suddenly off the throttle or hit the brakes (lightening the rear) and now they're still going too fast with a light rear - spin. When performance driving like on the track you can counteract the onset of slight oversteer by counter steering and gently applying slightly MORE throttle to transfer effective mass to the rear and increase grip for the lateral needs. There's a phrase "keep your foot in it" and it is not a natural reaction to go faster when you're going too fast. Understeer is how people think - if I'm going too fast I won't make the turn, I will need to slow down. If you're understeering and you lift to go for the brakes you may not save yourself, the tires only have so much grip, but at least the mass transfer won't make things worse.

Knowing this and actually doing it when you're caught by surprise are two different things. If you don't have muscle memory from training and track time, many will probably still do the wrong thing. Your mind simply isn't in "racing" mode when you're driving back from the theatre or heading for groceries. I have an OK amount of training but not the seat time I'd like to have. I always worry that I'll react in the wrong way even though I 'know better' if I was asked to describe what one should do in a given situation.

Increasing the size of your rear bar makes the car more neutral, but also puts you slightly more at risk of some of the above situations. The M5 rear bar still doesn't dial out all of the understeer from the E39 and I specifically mentioned my emergency avoidance moves to cite the fact that it doesn't suddenly become a spin-master. I will point out that I didn't touch the brakes during my one particularly wicked incident - there was no time and luckily my slow-mo brain knew not to under the circumstances. It was just like a movie - "Oh sh%*@!" and everything went to 1/4 speed for me, giving me time to think and react. Lucky. It could have just as easily gone "omg! *crunch*"

And, then who would buy an M5 if 540s

handled as good as an M5?

So it's a little of all above, a LOT of people do not want a stiffer handling car, it's safer for MOST people to have some understeer, and yeah if people only wanted M5 performance, no one would buy 525s, 530s, 540s etc..

Just in case some of you didn't know....(m)

When removing your old sway bar and install the new one you will need to put a thin wrench on the back side of the sway bar link (#6) to keep the stud from turning when you remove the (#7) nut.
Push back the rubber boot just behind the sway bar and you will see the two flats for the wrench to fit on.